The Variability Of Climate

The variability of the earth's climate is almost as extraordinary as its
uniformity. This variability is made up partly of a long, slow tendency
in one direction and partly of innumerable cycles of every conceivable
duration from days, or even hours, up to millions of years. Perhaps the
easiest way to grasp the full complexity of the matter is to put the
chief types of climatic sequence in the form of a table.

In assigning names to the various types an attempt has been made to
indicate something of the nature of the sequence so far as duration,
periodicity, and general tendencies are concerned. Not even the rich
English language of the twentieth century, however, furnishes words with
enough shades of meaning to express all that is desired. Moreover,
except in degree, there is no sharp distinction between some of the
related types, such as glacial fluctuations and historic pulsations.
Yet, taken as a whole, the table brings out the great contrast between
two absolutely diverse extremes. At the one end lies well-nigh eternal
uniformity, or an extremely slow progress in one direction throughout
countless ages; at the other, rapid and regular vibrations from day to
day, or else irregular and seemingly unsystematic vacillations due to
cyclonic storms, both of which types are repeated millions of times
during even a single glacial fluctuation.

The meaning of cosmic uniformity has been explained in the preceding
chapter. Its relation to the other types of climatic sequences seems to
be that it sets sharply defined limits beyond which no changes of any
kind have ever gone since life, as we know it, first began. Secular
progression, on the other hand, means that in spite of all manner of
variations, now this way and then the other, the normal climate of the
earth, if there is such a thing, has on the whole probably changed a
little, perhaps becoming more complex. After each period of continental
uplift and glaciation--for such are preeminently the times of
complexity--it is doubtful whether the earth has ever returned to quite
its former degree of monotony. Today the earth has swung away from the
great diversity of the glacial period. Yet we still have contrasts of
what seem to us great magnitude. In low depressions, such as Turfan in
the central deserts of Eurasia, the thermometer sometimes ranges from
0 deg.F. in the morning to 60 deg. in the shade at noon. On a cloudy day in the
Amazon forest close to the seashore, on the contrary, the temperature
for months may rise to 85 deg. by day and sink no lower than 75 deg. at night.

The reasons for the secular progression of the earth's climate appear to
be intimately connected with those which have caused the next, and, in
many respects, more important type of climatic sequence, which consists
of geological oscillations. Both the progression and the oscillations
seem to depend largely on three purely terrestrial factors: first, the
condition of the earth's interior, including both internal heat and
contraction; second, the salinity and movement of the ocean; and third,
the composition and amount of the atmosphere. To begin with the earth's
interior--its loss of heat appears to be an almost negligible factor in
explaining either secular progression or geologic oscillation. According
to both the nebular and the planetesimal hypotheses, the earth's crust
appears to be colder now than it was hundreds or thousands of millions
of years ago. The emission of internal heat, however, had probably
ceased to be of much climatic significance near the beginning of the
geological record, for in southern Canada glaciation occurred very early
in the Proterozoic era. On the other hand, the contraction of the earth
has produced remarkable effects throughout the whole of geological time.
It has lessened the earth's circumference by a thousand miles or more,
as appears from the way in which the rocks have been folded and thrust
bodily over one another. According to the laws of dynamics this must
have increased the speed of the earth's rotation, thus shortening the
day, and also having the more important effect of increasing the bulge
at the equator. On the other hand, recent investigations indicate that
tidal retardation has probably diminished the earth's rate of rotation
more than seemed probable a few years ago, thus lengthening the day and
diminishing the bulge at the equator. Thus two opposing forces have been
at work, one causing acceleration and one retardation. Their combined
effect may have been a factor in causing secular progression of climate.
It almost certainly was of much importance in causing pronounced
oscillations first one way and then the other. This matter, together
with most of those touched in these first chapters, will be expanded in
later parts of the book. On the whole the tendency appears to have been
to create climatic diversity in place of uniformity.

The increasing salinity of the oceans may have been another factor in
producing secular progression, although of slight importance in respect
to oscillations. While the oceans were still growing in volume, it is
generally assumed that they must have been almost fresh for a vast
period, although Chamberlin thinks that the change in salinity has been
much less than is usually supposed. So far as the early oceans were
fresher than those of today, their deep-sea circulation must have been
less hampered than now by the heavy saline water which is produced by
evaporation in warm regions. Although this saline water is warm, its
weight causes it to descend, instead of moving poleward in a surface
current; this descent slows up the rise of the cold water which has
moved along in the depths of the ocean from high latitudes, and thus
checks the general oceanic circulation. If the ancient oceans were
fresher and hence had a freer circulation than now, a more rapid
interchange of polar and equatorial water presumably tended to equalize
the climate of all latitudes.

Again, although the earth's atmosphere has probably changed far less
during geological times than was formerly supposed, its composition has
doubtless varied. The total volume of nitrogen has probably increased,
for that gas is so inert that when it once becomes a part of the air it
is almost sure to stay there. On the other hand, the proportions of
oxygen, carbon dioxide, and water vapor must have fluctuated. Oxygen is
taken out constantly by animals and by all the processes of rock
weathering, but on the other hand the supply is increased when plants
break up new carbon dioxide derived from volcanoes. As for the carbon
dioxide, it appears probable that in spite of the increased supply
furnished by volcanoes the great amounts of carbon which have gradually
been locked up in coal and limestone have appreciably depleted the
atmosphere. Water vapor also may be less abundant now than in the past,
for the presence of carbon dioxide raises the temperature a little and
thereby enables the air to hold more moisture. When the area of the
oceans has diminished, and this has recurred very often, this likewise
would tend to reduce the water vapor. Moreover, even a very slight
diminution in the amount of heat given off by the earth, or a decrease
in evaporation because of higher salinity in the oceans, would tend in
the same direction. Now carbon dioxide and water vapor both have a
strong blanketing effect whereby heat is prevented from leaving the
earth. Therefore, the probable reduction in the carbon dioxide and water
vapor of the earth's atmosphere has apparently tended to reduce the
climatic monotony and create diversity and complexity. Hence, in spite
of many reversals, the general tendency of changes, not only in the
earth's interior and in the oceans, but also in the atmosphere, appears
to be a secular progression from a relatively monotonous climate in
which the evolution of higher organic forms would scarcely be rapid to
an extremely diverse and complex climate highly favorable to progressive
evolution. The importance of these purely terrestrial agencies must not
be lost sight of when we come to discuss other agencies outside the
earth.

In Table 2 the next type of climatic sequence is geologic oscillation.
This means slow swings that last millions of years. At one extreme of
such an oscillation the climate all over the world is relatively
monotonous; it returns, as it were, toward the primeval conditions at
the beginning of the secular progression. At such times magnolias,
sequoias, figs, tree ferns, and many other types of subtropical plants
grew far north in places like Greenland, as is well known from their
fossil remains of middle Cenozoic time, for example. At these same
times, and also at many others before such high types of plants had
evolved, reef-making corals throve in great abundance in seas which
covered what is now Wisconsin, Michigan, Ontario, and other equally cool
regions. Today these regions have an average temperature of only about
70 deg.F. in the warmest month, and average well below freezing in winter.
No reef-making corals can now live where the temperature averages below
68 deg.F. The resemblance of the ancient corals to those of today makes it
highly probable that they were equally sensitive to low temperature.
Thus, in the mild portions of a geologic oscillation the climate seems
to have been so equable and uniform that many plants and animals could
live 1500 and at other times even 4000 miles farther from the equator
than now.

At such times the lands in middle and high latitudes were low and small,
and the oceans extended widely over the continental platforms. Thus
unhampered ocean currents had an opportunity to carry the heat of low
latitudes far toward the poles. Under such conditions, especially if the
conception of the great subequatorial continent of Gondwana land is
correct, the trade winds and the westerlies must have been stronger and
steadier than now. This would not only enable the westerlies, which are
really southwesterlies, to carry more heat than now to high latitudes,
but would still further strengthen the ocean currents. At the same time,
the air presumably contained an abundance of water vapor derived from
the broad oceans, and an abundance of atmospheric carbon dioxide
inherited from a preceding time when volcanoes contributed much carbon
dioxide to the air. These two constituents of the atmosphere may have
exercised a pronounced blanketing effect whereby the heat of the earth
with its long wave lengths was kept in, although the energy of the sun
with its shorter wave lengths was not markedly kept out. Thus everything
may have combined to produce mild conditions in high latitudes, and to
diminish the contrast between equator and pole, and between summer and
winter.

Such conditions perhaps carry in themselves the seeds of decay. At any
rate while the lands lie quiet during a period of mild climate great
strains must accumulate in the crust because of the earth's contraction
and tidal retardation. At the same time the great abundance of plants
upon the lowlying plains with their mild climates, and the marine
creatures upon the broad continental platforms, deplete the atmospheric
carbon dioxide. Part of this is locked up as coal and part as limestone
derived from marine plants as well as animals. Then something happens so
that the strains and stresses of the crust are released. The sea floors
sink; the continents become relatively high and large; mountain ranges
are formed; and the former plains and emergent portions of the
continental platforms are eroded into hills and valleys. The large size
of the continents tends to create deserts and other types of climatic
diversity; the presence of mountain ranges checks the free flow of winds
and also creates diversity; the ocean currents are likewise checked,
altered, and diverted so that the flow of heat from low to high
latitudes is diminished. At the same time evaporation from the ocean
diminishes so that a decrease in water vapor combines with the previous
depletion of carbon dioxide to reduce the blanketing effect of the
atmosphere. Thus upon periods of mild monotony there supervene periods
of complexity, diversity, and severity. Turn to Table 1 and see how a
glacial climate again and again succeeds a time when relative mildness
prevailed almost everywhere. Or examine Fig. 1 and notice how the lines
representing temperatures go up and down. In the figure Schuchert makes
it clear that when the lands have been large and mountain-making has
been important, as shown by the high parts of the lower shaded area, the
climate has been severe, as shown by the descent of the snow line, the
upper shaded area. In the diagram the climatic oscillations appear
short, but this is merely because they have been crowded together,
especially in the left hand or early part. There an inch in length may
represent a hundred million years. Even at the right-hand end an inch is
equivalent to several million years.

The severe part of a climatic oscillation, as well as the mild part,
will be shown in later chapters to bear in itself certain probable seeds
of decay. While the lands are being uplifted, volcanic activity is
likely to be vigorous and to add carbon dioxide to the air. Later, as
the mountains are worn down by the many agencies of water, wind, ice,
and chemical decay, although much carbon dioxide is locked up by the
carbonation of the rocks, the carbon locked up in the coal is set free
and increases the carbon dioxide of the air. At the same time the
continents settle slowly downward, for the earth's crust though rigid as
steel is nevertheless slightly viscous and will flow if subjected to
sufficiently great and enduring pressure. The area from which
evaporation can take place is thereby increased because of the spread of
the oceans over the continents, and water vapor joins with the carbon
dioxide to blanket the earth and thus tends to keep it uniformly warm.
Moreover, the diminution of the lands frees the ocean currents from
restraint and permits them to flow more freely from low latitudes to
high. Thus in the course of millions of years there is a return toward
monotony. Ultimately, however, new stresses accumulate in the earth's
crust, and the way is prepared for another great oscillation. Perhaps
the setting free of the stresses takes place simply because the strain
at last becomes irresistible. It is also possible, as we shall see, that
an external agency sometimes adds to the strain and thereby determines
the time at which a new oscillation shall begin.

In Table 2 the types of climatic sequences which follow "geologic
oscillations" are "glacial fluctuations," "orbital precessions" and
"historical pulsations." Glacial fluctuations and historical pulsations
appear to be of the same type, except as to severity and duration, and
hence may be considered together. They will be treated briefly here
because the theories as to their causes are outlined in the next two
chapters. Oddly enough, although the historic pulsations lie much closer
to us than do the glacial fluctuations, they were not discovered until
two or three generations later, and are still much less known. The most
important feature of both sequences is the swing from a glacial to an
inter-glacial epoch or from the arsis or accentuated part of an
historical pulsation to the thesis or unaccented part. In a glacial
epoch or in the arsis of an historic pulsation, storms are usually
abundant and severe, the mean temperature is lower than usual, snow
accumulates in high latitudes or upon lofty mountains. For example, in
the last such period during the fourteenth century, great floods and
droughts occurred alternately around the North Sea; it was several times
possible to cross the Baltic Sea from Germany to Sweden on the ice, and
the ice of Greenland advanced so much that shore ice caused the Norsemen
to change their sailing route between Iceland and the Norse colonies in
southern Greenland. At the same time in low latitudes and in parts of
the continental interior there is a tendency toward diminished rainfall
and even toward aridity and the formation of deserts. In Yucatan, for
example, a diminution in tropical rainfall in the fourteenth century
seems to have given the Mayas a last opportunity for a revival of their
decaying civilization.

(After Schuchert, in The Evolution of the Earth and Its Inhabitants,
edited by R. S. Lull.) Diagram showing the times and probable extent of
the more or less marked climate changes in the geologic history of North
America, and of its elevation into chains of mountains.]

Among the climatic sequences, glacial fluctuations are perhaps of the
most vital import from the standpoint of organic evolution; from the
standpoint of human history the same is true of climatic pulsations.
Glacial epochs have repeatedly wiped out thousands upon thousands of
species and played a part in the origin of entirely new types of plants
and animals. This is best seen when the life of the Pennsylvanian is
contrasted with that of the Permian. An historic pulsation may wipe out
an entire civilization and permit a new one to grow up with a radically
different character. Hence it is not strange that the causes of such
climatic phenomena have been discussed with extraordinary vigor. In few
realms of science has there been a more imposing or more interesting
array of theories. In this book we shall consider the more important of
these theories. A new solar or cyclonic hypothesis and the hypothesis of
changes in the form and altitude of the land will receive the most
attention, but the other chief hypotheses are outlined in the next
chapter, and are frequently referred to throughout the volume.

Between glacial fluctuations and historical pulsations in duration, but
probably less severe than either, come orbital precessions. These stand
in a group by themselves and are more akin to seasonal alternations than
to any other type of climatic sequence. They must have occurred with
absolute regularity ever since the earth began to revolve around the sun
in its present elliptical orbit. Since the orbit is elliptical and since
the sun is in one of the two foci of the ellipse, the earth's distance
from the sun varies. At present the earth is nearest the sun in the
northern winter. Hence the rigor of winter in the northern hemisphere is
mitigated, while that of the southern hemisphere is increased. In about
ten thousand years this condition will be reversed, and in another ten
thousand the present conditions will return once more. Such climatic
precessions, as we may here call them, must have occurred unnumbered
times in the past, but they do not appear to have been large enough to
leave in the fossils of the rocks any traces that can be distinguished
from those of other climatic sequences.

We come now to Brueckner periods and sunspot cycles. The Brueckner periods
have a length of about thirty-three years. Their existence was suggested
at least as long ago as the days of Sir Francis Bacon, whose statement
about them is quoted on the flyleaf of this book. They have since been
detected by a careful study of the records of the time of harvest,
vintage, the opening of rivers to navigation, and the rise or fall of
lakes like the Caspian Sea. In his book on Klimaschwankungen seit
1700, Brueckner has collected an uncommonly interesting assortment of
facts as to the climate of Europe for more than two centuries. More
recently, by a study of the rate of growth of trees, Douglass, in his
book on Climatic Cycles and Tree Growth, has carried the subject still
further. In general the nature of the 33-year periods seems to be
identical with that of the 11- or 12- year sunspot cycle, on the one
hand, and of historic pulsations on the other. For a century observers
have noted that the variations in the weather which everyone notices
from year to year seem to have some relation to sunspots. For
generations, however, the relationship was discussed without leading to
any definite conclusion. The trouble was that the same change was
supposed to take place in all parts of the world. Hence, when every sort
of change was found somewhere at any given sunspot stage, it seemed as
though there could not be a relationship. Of late years, however, the
matter has become fairly clear. The chief conclusions are, first, that
when sunspots are numerous the average temperature of the earth's
surface is lower than normal. This does not mean that all parts are
cooler, for while certain large areas grow cool, others of less extent
become warm at times of many sunspots. Second, at times of many sunspots
storms are more abundant than usual, but are also confined somewhat
closely to certain limited tracks so that elsewhere a diminution of
storminess may be noted. This whole question is discussed so fully in
Earth and Sun that it need not detain us further in this preliminary
view of the whole problem of climate. Suffice it to say that a study of
the sunspot cycle leads to the conclusion that it furnishes a clue to
many of the unsolved problems of the climate of the past, as well as a
key to prediction of the future.

Passing by the seasonal alternations which are fully explained as the
result of the revolution of the earth around the sun, we may merely
point out that, like the daily vibrations which bring Table 2 to a
close, they emphasize the outstanding fact that the main control of
terrestrial climate is the amount of energy received from the sun. This
same principle is illustrated by pleionian migrations. The term "pleion"
comes from a Greek word meaning "more." It was taken by Arctowski to
designate areas or periods where there is an excess of some climatic
element, such as atmospheric pressure, rainfall, or temperature. Even if
the effect of the seasons is eliminated, it appears that the course of
these various elements does not run smoothly. As everyone knows, a
period like the autumn of 1920 in the eastern United States may be
unusually warm, while a succeeding period may be unseasonably cool.
These departures from the normal show a certain rough periodicity. For
example, there is evidence of a period of about twenty-seven days,
corresponding to the sun's rotation and formerly supposed to be due to
the moon's revolution which occupies almost the same length of time.
Still other periods appear to have an average duration of about three
months and of between two and three years. Two remarkable discoveries
have recently been made in respect to such pleions. One is that a given
type of change usually occurs simultaneously in a number of well-defined
but widely separated centers, while a change of an opposite character
arises in another equally well-defined, but quite different, set of
centers. In general, areas of high pressure have one type of change and
areas of low pressure the other type. So systematic are these
relationships and so completely do they harmonize in widely separated
parts of the earth, that it seems certain that they must be due to some
outside cause, which in all probability can be only the sun. The second
discovery is that pleions, when once formed, travel irregularly along
the earth's surface. Their paths have not yet been worked out in detail,
but a general migration seems well established. Because of this, it is
probable that if unusually warm weather prevails in one part of a
continent at a given time, the "thermo-pleion," or excess of heat, will
not vanish but will gradually move away in some particular direction. If
we knew the path that it would follow we might predict the general
temperature along its course for some months in advance. The paths are
often irregular, and the pleions frequently show a tendency to break up
or suddenly revive. Probably this tendency is due to variations in the
sun. When the sun is highly variable, the pleions are numerous and
strong, and extremes of weather are frequent. Taken as a whole the
pleions offer one of the most interesting and hopeful fields not only
for the student of the causes of climatic variations, but for the man
who is interested in the practical question of long-range weather
forecasts. Like many other climatic phenomena they seem to represent the
combined effect of conditions in the sun and upon the earth itself.

The last of the climatic sequences which require explanation is the
cyclonic vacillations. These are familiar to everyone, for they are the
changes of weather which occur at intervals of a few days, or a week or
two, at all seasons, in large parts of the United States, Europe, Japan,
and some of the other progressive parts of the earth. They do not,
however, occur with great frequency in equatorial regions, deserts, and
many other regions. Up to the end of the last century, it was generally
supposed that cyclonic storms were purely terrestrial in origin. Without
any adequate investigation it was assumed that all irregularities in the
planetary circulation of the winds arise from an irregular distribution
of heat due to conditions within or upon the earth itself. These
irregularities were supposed to produce cyclonic storms in certain
limited belts, but not in most parts of the world. Today this view is
being rapidly modified. Undoubtedly, the irregularities due to purely
terrestrial conditions are one of the chief contributory causes of
storms, but it begins to appear that solar variations also play a part.
It has been found, for example, that not only the mean temperature of
the earth's surface varies in harmony with the sunspot cycle, but that
the frequency and severity of storms vary in the same way. Moreover, it
has been demonstrated that the sun's radiation is not constant, but is
subject to innumerable variations. This does not mean that the sun's
general temperature varies, but merely that at some times heated gases
are ejected rapidly to high levels so that a sudden wave of energy
strikes the earth. Thus, the present tendency is to believe that the
cyclonic variations, the changes of weather which come and go in such a
haphazard, irresponsible way, are partly due to causes pertaining to the
earth itself and partly to the sun.

From this rapid survey of the types of climatic sequences, it is evident
that they may be divided into four great groups. First comes cosmic
uniformity, one of the most marvelous and incomprehensible of all known
facts. We simply have no explanation which is in any respect adequate.
Next come secular progression and geologic oscillations, two types of
change which seem to be due mainly to purely terrestrial causes, that
is, to changes in the lands, the oceans, and the air. The general
tendency of these changes is toward complexity and diversity, thus
producing progression, but they are subject to frequent reversals which
give rise to oscillations lasting millions of years. The processes by
which the oscillations take place are fully discussed in this book.
Nevertheless, because they are fairly well understood, they are deferred
until after the third group of sequences has been discussed. This group
includes glacial fluctuations, historic pulsations, Brueckner periods,
sunspot cycles, pleionian migrations, and cyclonic vacillations. The
outstanding fact in regard to all of these is that while they are
greatly modified by purely terrestrial conditions, they seem to owe
their origin to variations in the sun. They form the chief subject of
Earth and Sun and in their larger phases are the most important topic
of this book also. The last group of sequences includes orbital
precessions, seasonal alternations, and daily variations. These may be
regarded as purely solar in origin. Yet their influence, like that of
each of the other groups, is much modified by the earth's own
conditions. Our main problem is to separate and explain the two great
elements in climatic changes,--the effects of the sun, on the one hand,
and of the earth on the other.